1. Field of Invention
This invention relates generally to masonry anchors, and more particularly to a self-tapping, screw-type masonry anchor which can readily be turned into a hole drilled in masonry to secure a fixture or other object thereto, the anchor also being capable of being chemically retained in the hole whereby the installed anchor which combines the holding power of a self-tapping anchor with that of a chemical anchor is exceptionally resistant to forces seeking to pull the anchor out of the hole as well as to vibratory and shock forces.
2. Status of Prior Art
It is frequently necessary to secure fixtures, brackets, channel pieces and other more or less heavy objects to the surface of a masonry structure formed of concrete, brickwork or other masonry material. These objects are fastened to the masonry structure by masonry anchors.
Concrete is made by mixing cement and an aggregate of sand or broken stone screenings with gravel. Compressive strength is generally accepted as the principal index to the structural quality of concrete. Mixtures for concrete masonry structures, such as walls and partitions, ordinarily employ aggregates having a maximum size of one-half inch. Masonry brick is usually formed from clay, shale or pumice hardened by heat. Bricks for this purpose are available in different degrees of hardness, depending on the material used in making the brick.
But regardless of the material used in forming a masonry structure, when a hole is drilled therein to receive a self-tapping metal masonry anchor whose threaded shank taps the bank of the hole, because the material is friable this tapping action may cause the masonry to crumble or pulverize. Hence the suitability of a self-tapping masonry anchor depends on the quality of the masonry.
The Ernst et al. patent 3,937,119, discloses a self-tapping, screw-type masonry anchor having a sharp-crested helical male thread surrounding the shank, the convolutions of the thread running the length of the shank. When the anchor is screwed into a hole drilled in masonry, the male thread functions to tap the bank of the hole to create an internal female thread therein. Since the male thread on the shank mates with the female thread in the bank of the hole, the anchor is then mechanically resistant to pull-out forces which seek to back the anchor out of the hole.
Also included in the Ernst et al. anchor is a second thread whose convolutions surround the shank in the zone between successive convolutions of the cutting thread. The crests of the second thread have a diameter smaller than that the crests of the cutting thread and about equal to the diameter of the masonry hole. This second thread functions as a guide thread to center the anchor in the masonry hole so that the anchor is not permitted to tilt as it is turned into the hole. By preventing such tilting, one enhances the pull-out resistance of the anchor; for if the anchor were tilted, the crests of the cutting thread would then not be properly embedded in the hole.
Other patents disclosing a screw-type anchor which includes a secondary thread in addition to a primary cutting thread are the patents to Dixon et al. 5,051,136 and 5,628,161. As in the Ernst et al. patent, the secondary thread in the Dixon et al. patents serve to guide the screw anchor and prevent it from tilting.
The Giannuzzi patent 5,282,708 discloses a self-tapping screw-type thread in which the crests of the male thread decrease progressively in diameter from the leading to the trailing end to create a reverse taper. But there is no secondary thread disclosed in this patent.
Also of prior art interest in regard to screw-type masonry anchors is the European patent application No. 91 919296.3 of Charles Bickford which discloses an anchor in which the successive convolutions of the cutting thread which has a helix angle of at least 8 degrees, are spaced apart by an intervening land having a helical zone depressed therein.
In any form of screw-type masonry anchor, one is faced with the problem that a cutting action by the male cutting thread to cut a deep female thread in the bank of the drilled masonry hole may not have the desired result despite the fact that the deeper the female thread, the greater is the holding power of the anchor. The reason this result is not always realized is that the cutting action of the male thread may so disintegrate the masonry material as to weaken the holding power of the anchor.
In order, therefore, to anchor a threaded bolt or stud in a hole drilled in masonry without disintegrating the masonry, it is common practice to use a curable chemical bonding agent or adhesive for this purpose as disclosed, for example, in the Giannuzzi patent 5,628,161 which shows a chemically-bonded stud anchor for fastening a fixture to masonry.
A typical bonding agent of this type has two flowable components, one being a resinous bonding agent, the other a hardener therefor. The resins may be phenol, vinyl, ester or epoxy based. The two components, when stored, must be separated to prevent interaction therebetween. Many bonding agents currently available have an accelerated curing time and set within 30 minutes to afford substantial holding power. In practice, a charge of the resinous component and sufficient hardener intermixed therewith are deposited in the hole, and a threaded mounting stud is then inserted in the hole.
To this end, use may be made of a dispenser gun to inject a charge of the flowable bonding agent into the drilled hole. Or the charge may be contained in a capsule that is deposited in the drilled hole and is ruptured to release the bonding agent when the stud is inserted in the hole.
After the resinous interfacial layer between the stud and the bank of the hole cures and rigidifies, it then bonds itself both to the stud and to the masonry whereby the stud is permanently anchored in the hole. In order now to secure a fixture to the masonry face, the fixture which has a mounting hole therein is placed over and onto the projecting stud and locked thereto by a washer and nut.
It is known in a chemical anchor to coat the externally-threaded cylindrical shank of the stud or bolt with a release agent, such as TEFLON (PTFE) or a similar material having non-stick properties. When the shank of the bolt is inserted in a drilled masonry hole having a deposit of an uncured adhesive therein, the release coating prevents this adhesive, when cured and hardened, from adhering to the shank.
A chemical anchor of this type is disclosed in the Kellison patent 4,642,964 and also in the Roth patent 5,590,360. In the chemical anchors disclosed in these patents, when the epoxy cures and hardens in the drilled masonry hole, a female thread is then impressed in the hardened epoxy surrounding the shank. This female thread matches the male thread of the release-coated shank; hence the bolt is then free to turn in either direction.
A serious practical drawback of a chemical masonry anchor is that it does not function as such until the bonding agent has fully cured and hardened so that the anchor is firmly retained in the masonry hole. Even with fast-acting epoxy bonding agents, it takes at least a half hour before the installation is fully secure. If therefore a chemical anchor is used to fasten a railing or a fixture to the surface of the masonry, it becomes necessary to provide some external means to maintain the railing or fixture in its proper position until the bonding agent has fully hardened. In contradistinction, with a screw-type anchor for fastening an object to masonry, once screwed into the drilled masonry hole and tightened, it attains its full holding power.
Another drawback of a chemical anchor for fastening an object to masonry is that after the bonding agent has hardened, it is usually then necessary to effect tightening of the fastening by turning in the anchor to further advance it into the hardened agent. It becomes necessary therefore as in the Kellison patent '964 to create a free space in the bonding agent below the shank of the anchor to permit the shank of the anchor to advance.
Since a hybrid masonry anchor, in accordance with the invention, acts chemically as well as mechanically to retain the anchor turned into a masonry hole, of prior art interest is the Gugle patent 3,897,713. Though this patent is not related to the field of masonry anchors, it does disclose a stud type anchor having a dual thread that is both chemically and mechanically held in a hole formed in a plastic workpiece.
In Gugle, the stud is provided with a shank having a high-crested helical cutting thread that taps its way into the hole in the workpiece and thereby mechanically anchors the stud, the shank being coated with a heat-activatable adhesive. The shank is also provided with a smaller crested thread that does not engage the bank of the hole, but acts only to increase the area of surface contact between the shank of the stud and the heat-activatable adhesive coating the shank. In order to activate this adhesive and chemically anchor the stud, heat must be applied to the workpiece to melt the coating on the shank.
When a fixture or other object is fastened to masonry by means of an anchor that goes into a drilled masonry hole through a mounting hole in the fixture that lies in registration with the drilled hole, it is useful to be able not only to fasten the object to the masonry, but also to be able to later unfasten the object. Thus if the object fastened to the masonry is a heavy duty motor, one must be able to detach the motor when it becomes necessary to replace it.
With a screw-type masonry anchor which taps a female thread in the bank of the masonry hole, it is a simple matter to unscrew the anchor to unfasten the motor. But with a chemical anchor, should one wish to be able to later remove this anchor from the masonry hole, it is essential that the bonding agent, though serving to chemically retain the shank of the anchor in the masonry hole, not be bonded to the shank, thereby permitting the bolt or stud to be unscrewed from the hardened bonding agent.